1. Field of the Invention
The present invention relates generally to a power amplifier and a modulator thereof, and more particularly, to a sigma-delta pulse-width modulation class-D power amplifier with an automatic frequency modulation function and a modulator thereof.
2. Description of Related Art
Power amplifiers can be classified into many types based on their applications, such as class-A, class-B, class-AB, class-C, class-D, and so on. For example, the class-D power amplifiers are being widely used in audio signal processing of handheld and mobile devices due to their high power conversion efficiencies (higher than 90%). Some class-D amplifiers utilize a pulse width modulator to generate a continuous pulse where the pulse widths vary with the amplitude of audio signals to control operation of a switching circuit in the class-D power amplifiers. However, the performance of the class-D power amplifiers on those products that are rather reluctant to have a high signal distortion is not as good as that of the class-AB power amplifiers.
In order to prevent nonlinear distortion of the output signal of the class-D power amplifier, a sigma-delta class-D power amplifier has therefore been proposed which can ensure a lower signal distortion than the class-AB power amplifier while the high power conversion efficiency of the class-D power amplifier is still maintained. This makes the sigma-delta class-D power amplifiers very competitive in the market. However, the sigma-delta class-D power amplifier has a critical shortcoming, i.e., when the input signal becomes larger to a certain extent, typically a half of a reference level, the total harmonic distortion plus noise (THD+N) of the sigma-delta class-D power amplifier will rise sharply. The term THD+N used herein is defined as a ratio of the sum of the powers of all the harmonic distortion signals presented by the device itself and noises to the output power.
In order to overcome the critical shortcoming of the sigma-delta class-D power amplifier, a sigma-delta pulse-width modulation class-D power amplifier has been developed accordingly. The sigma-delta pulse-width modulation class-D power amplifier can overcome the sharp rise problem of the THD+N of the sigma-delta class-D power amplifier while the power conversion efficiency can still be maintained in common power applications. In addition, this technology utilizes the principle of continuous-time sigma-delta modulation. Therefore, passive components, such as, resistors, capacitors, and so forth, are required in a loop filter circuit.
However, due to limitations in the fabrication technology, the absolute values of the passive components have a variation of about ±20%˜±40%, which significantly affects the circuit performance. The process variations of the passive components cannot be precisely controlled such that the circuit performance variation caused by the variation of the absolute values of the passive components can be unduly large. As a result, the output signal quality of the sigma-delta pulse-width modulation class-D power amplifier cannot be effectively increased.